Such heating apparatus are counted as belonging to the state of the art and are typically applied, in order one the one hand to provide the heat necessary for the room heating and on the other hand to provide the heat necessary for the warm water provision. In a primary heat exchanger, water is heated typically by way of a combustion process or also way of other suitable heat sources, in order then to be fed to a heating circuit for the room heating or a heating circuit for the provision of warm water. Thereby, it is typically the case of circulation systems which are activated by way of a switch-over valve. In normal operation, the heating circuit for the room heating is connected to the primary heat exchanger. If the heating of domestic water (service water) is necessary, which for example is triggered by way of opening a tapping valve, then the switch-over valve re-switches, so that the secondary heat exchanger for the heating of domestic water is connected to the primary heat exchanger.
For reasons of safety, one must ensure that the primary heat exchanger is always incorporated into an open cycle, in order to be able to lead away heat occurring there and to reliably prevent an overheating, in particular an evaporation of the water located there. Basically, such a cycle is always formed by the heating circuit for by the room heating, but however this can also be temporarily interrupted, for example with a closure of the thermostat valve on the radiators. A bypass conduit is provided for this case, with which one can ensure that the primary heat exchanger is always incorporated into an interconnected cycle.
For this purpose, it is counted as belonging to the state of the art, to provide a bypass conduit which is arranged parallel to the heating circuits and which connects the feed (flow) and return of the primary heat exchanger to one another, and via which a typically pressure-controlled value can be activate and deactivated. If therefore, due to further supply of heat at the primary heat exchanger, the pressure within the feed conduit (also called flow conduit) rises, on exceeding a predefined pressure, the bypass valve opens and thus the bypass conduit which hydraulically connects the feed and return to one another, is released.